Rework unlockables display: show those initially locked, grouped by threshold.

Also adjust sizes and use the background colour for disabled items.

Items are shown in groups, using the full width (or height) of the box and
with equal spacing between groups.

Aligning the icons with the threshold marks leaves much more scope for
overlap; and with commit 91751a3 merged, they do indeed overlap unless
shrunk – which would leave them too small.